Process for making anhydrides



Nov. 10, 1953 RIGON 2,658,914

PROCESS FOR MAKING ANHYDRIDES Filed June 28, 1950 ANHYDRIDE. ,Aclo,

Couoznsee WATflR AND Exccss OXYGEN CONTAINING GAs DILUENT AND CATALYSTCOOLER ALDEHYDE WASHING COLUMN Ann-womb:

GAS AND B owma POM OX\DIZ\NG AND SOL-UTE HEAT Excmmc:

CLEAN SOLVENT OXYGEN CONTAINING GAS INVENTOR. Ll NO RIGON.

BY I

Patented Nov. 10, 1953 UNITED STATES PATENT OFFICE 7 r 2,658,914 7 1PROCESS FOR MAKlNG-ANHYDRIDES Lino RigonlMelle, France, assignor to LesUsines de Melle (Societe 'Anonyme), Saint-Leger-les- Melle, France, acorporation of France 1 npplication'junen, 1950, Serial No. 170,714

This invention relates to a process for making aliphatic anhydrides.More particularly, it relates to a continuous processfor the productionof lower aliphatic anhydridesby which expression I mean anhydrides whichhave from 4 to 8 carbon atomssuch as acetic anhydride, but withoutlimitatiomby direct oxidation with a gas containing molecular oxygen ofthe corresponding aldehydes.-- 7 r i is The principal object of theinvention is to provide a simple, eflicient process of the kinddescribed to produceahigh yield of anhydride at a low cost of operationand employingsimple apparatusp r The invention accordingly comprises'thenovel processes and steps-of processes, specific embodiments of whicharedes'cribed hereinafter byway of example andin accordance with which Inow" prefer to practicethe invention. l I I have foundin accordancewithfmy'invention thatIV-may achieve the above objects by continuouslyintroducing a gas'con'taining molecular oxygen anda lower..aliphaticaldehyde such 7 as acetaldehyde but without. .limitation, into a liquidbath .containing the' followingingredients: The aldehyde to beoxidized,the corresponding acid,v thecacetic or other -.anhydride to beproduced, and metallic saltscapablepf acting as aldehyde oxidationcatalyst.- 'Thebath also contains one or more diluents havinga boilingpoint above 200 C. The diluent is present in the proportion of at least75% ofthe total weight of the bath and is chemically inert to the otheringredients thereof and to the oxygencontaining gas at temperaturesrequired for oxidation of the aldehydeinto anhydride and at suchtemperatures remains liquid. The diluent is also insoluble in and doesnot dissolve water and is miscible with, the anhydride to be producedand the corresponding acid. The mixtures of the aforementionedingredients are such as to be able to dissolve the'c'atalyst in'therequired proportions for the oxidation reaction. Diluents having suchproperties are referred to in the appended claims as inert diluents."The proportions of the ingredients are such that the bath is not of toohigh viscosity to interfere with efficient contact between gas andliquid. Under such conditions, when aldehyde and theoxygen containinggas are introduced into the bath,

anhydride is produced and there is no substantial combination of waterwith the formed anhydride to impair the yield of anhydride.; Under theconditions, of -my process, it-is also unnecessaryto return diluent oranhydride con- Claimspriority, application France my 15,1949

11 Claims. (01. 260-754 6) l V 2 tinuously to the liquid oxidation bath.Of course, it should be understood that instead of using alsinglealdehyde, there may be used two aldehydes simultaneously if. it isdesired to obtain a mixed anhydride of thekind R-CO-O-OC-R The diluentsmentioned above have a low degreeof volatilityl They increaseconsiderably the relative volatility of the water in the oxidation bathand, being present in the high propor-- tion of at least three-fourthsof the bath, cause the water to be carried away as itis formed by thegas stream passing through the bath; the diluent itself remaining in thebath. 7

As it is not compulsory for the auxiliary substances used as diluents inaccordance with this invention to assume another function, generallymore important than that of the diluent, the disadvantages which wouldresult from the assuming of more than one function by the auxile iarysubstances do not occur. The proportionof diluent or mixture of-diluents answering the above requirements will be preferably of to ofthe total weight of the bath. i

- There is 'noiup'per limit regardingthe boiling being monobasic. orpolybasic, reacted-with V monobasicl'or polybasic alcoholsor phenols. IThe oxidation reaction-is carried out atia temperaturein the range40-100 t'C., according to'thekind of anhydridefito'be produced; for in'-stance; temperature must be higher for making butyric anhydride'frombutyraldehyde than' for making acetic anhydride from'acetaldehyde- The;catalysts to be. employed are preferably metallic salts, moreparticularly salts of -polyvaIent. metals (such as-cobalt'; nickel,-copper,

vanadium)-and an acid such as acetic acid corresponding; to; theanhydridec to be produced;

however, other metallic salts,such assilvel? salts, ma 1 be u ed; h ssalts aytb Ema ployed either separately orin -mix tures,-ata; v 1 c ncer tio if in e o 1%;: er,a

erably between 23 and 60 mm. Hg-fiwiiicli cor-- responds, when theoperation is carried oufi at atmospheric pressure, to a propgrtioncfoxygend i lower than preferably b tween 3 and 8% by volume. Formation ofperoxides, such as per acetic acid, is thus avoided, therefore risksiorplosion are reduced. Furthermore, higheryields are secured since, theoxidation being less intensive, the. proportion of by-products resultingeither. from; decomposition of'the unstablepel oxi s or e v m h exi e onth s arting aldehyde, isreduced.

I have further found that it is advisable" to maintain theconcentrationof aldehyde in the oxidation bath below about 5% by weight,but not below about 0.3%. .IIlpIdQtlOfi, the concenna io a eh de-w l aain md etw n about l:and-.about-2%. of:the;-vveight otthe-bath.

It; is;preferable that the gases and vapors. escaping from the bath becooled atatemperature not ;exceeding. .0. approximately; at least partofJtheuncQndenSedga eS being retu ned 0 he oxidation apparatus. The-rateof flow ofthe gases-may. with advantage; beof lto 3 cubic meters,preferably-l8: to. 2.5. cubic meters,.per hourperiiterof bath;

The; operation maybe carried. out. either. at atmospheric: pressure or:at. a... h her: onlower pressure:

The process of thisrinven ion. may bea v aeouslyhecxecuted. inthefollowingmanner; using the: .a-ppara-tus.- diagrammatically representedin theiappendeddrawing which is notilimitative.

The aldehyde to be oxidized, for example; acetaldehyde; is:continuously.- supplied; through pipe Zzto; aicylindricalsreactionvesselljprovided, with adequate means for; putting gases into. intimatecontact with liquids; a-porous plate for example. ThQ-hGfltLBVOlYEdjbYithe reaction .is. removed :by any means whatever, for example,-. an;external coolingcdevicejcthroughwhich a portionsof, the oxidation-bathcontained in vesselll is. continu ouslyxcirculated bypump 3. Pipe-26permitsthe introduction vinto vessel. I ot the diluent: and; of the.catalysts...

Thezoxidizing; gases. are introduced through pipe. .l hand; after.having given the aldehyde .part

of their oxygen, escapethrough;pip.e;5iwhi1e.car-

rying. away both the reaction products .(anhydxida. aciclawaterl and:part oi. theeunconverted aldehyde Then, thegases. pass successivelythrough twocondensers Sand :1 cooled'bymeans of water and brine,respectively, The greatest part otthe: ent'rained'vapors iscondensedrthen separated from theresidual gases-.in separation chamber8; A liquid is obtained which consists oiE-acetic anhydride, aceticacid, water and'alde h-"y de.- From this material, the acetic orotherarr-hydride is separated and recoveredby-known means} as, for example;that described in United States Patent'No. 2,159,146. a

After separation from thecondensedliquid in I chamber 8} theresiduar-gases escape from'sepa pipe [6".-

ration chamber 8 through pipe l0 and are divided into two parts; thegreatest part flows through pipe H and is returned to vessel l by blowerl2 and pipe l3; the other part, corresponding to the discharge of anamount of nitrogen or other inertigasi canal to that-.suppliedduring thesame time as oxygen-containing gas, which discharge is necessary forkeeping the desired proportion of oxygen in the circulating gaseousmixture, is di- 19,- rected.through.. pipe l4 into washing column i5 inwhich itis scrubbed by a solvent which preferablyiissthazsameasuhstanceas the diluent supplied towesseh I at the start, but may be anothersubstance=such, for example, as the same aslthatt to be obtained, or thecorresponding acid; or a high boiling ester other than the? diluent;-ora generally any suitable solvent whateventhe use of which cannotinterfere with the process. The scrubbing solvent dissolves thealdehyde, acid and anhydride that may have remained-in the-gasesaftercondensation, and the gases; completely freedfrom the vaporstheywere carrying escape intothe' atmosphere through k solvent flowingout ofcolumn I5'is sent into heatexchanger i8; then into heater 9; then intocolumn l fl. 'Atthe foot of column-; fresh air (or otheroxygen-containing gas-y intended to, keep the desired concentration ofoxygen in the. circulating gaseous mixture, is; introduced throughpipeZl: This oxygen-containing gas in column 20 becomes-'laden'with'theproducts dissolved by the solvent in column (5-, and "is then sent m ehPipe 2? n h d ieaga ecus cycle; preferablyinto pipe I I5 he pl e new?tra a d ac nd m hydride; flows out of column" 2P0 through a, pipe 23* atthe foot thereof; and is sent by pump 24 into theheatl exchanger]?-where it givesits heatto the solvent: flowing out oi column 15, thenpasses into cooler "25' where it is broughtto atemperature:'suitab1e'-for the scrubbing; and then goes. intocoiumn'ljthroughpipe l1.

The iollowing. are examples'of' my processe 'eiew' Pr t ra t e it It ist n t e that theseexamplesare illustrativ of th invention and thii-ttheinvention is not" to is-consul: credaarestrieted thereto exceptasindicated in heenri n dnlaims Examples 1 i tr ucen .1.8 eac 5. 1 9lrgsoi .butyl benaoate, 16 k'gs. oi acetic anhydride, eggs, of aceticacid inwhich have been dissolved: 59 ram f 9 .a ais -li .00- g am of "cper acetate, and grains. oifnick el acetate, and 1.5 lies. ofacetaldeliyde.'

1This.mixture was brought to 5.5 CI, and a ar as. I; w s. .i itrcci cdgth remt h ra or .cubic rnetersjper hour. As soon as the ox; ation. 1ction'bega'n thisfactis recognis ab w l? r he em ra re th ba o "sees dowt k p he t mpe aiturea 'e" C1 aceta dehva w s 'a 'th at 'oifZO 5*kgs.peuhourr The reactionproducts e-. 7 iz h r' sidaalra s w n s dt bymeans" of "cooling until their temperature was brought down lac-"59C andwereseparated, and "part of the-residualQases was returned to theoxidation vessel and c irculated through the cycle, as above described;at the rate ofl25cubic-meters per hour. a Theequilihriumbeing attained;the concentration =-of; {oxygen the gases: introduced into the 7 "bathremained equal to 7=% by volume, and th'e 1 .al h q Transformation= rate52 concentrations of tile component's cf the path; equaltothe start ing"concentrations.-

*1 then ascertained-that the" condensate as obtained in separationchamber whad the following composition-:

Per cent by weight Acetic a ydn deeaees 5 Acetic acid 22.2 Water '10=Acetaldehyde 1 1158 This composition corresponds to transformae tionrate of 84% of the starting dehydej theanhydride yield waS-ZB 7a ofthe.ox dizedhldehyde.

2. I introducedintbthereaction. vessel;

78. .kes- .Q imeth l pbtha e. 20"kgs; of acetic miydriue; 2 kgs. ofacetic acid, 2 has f acetaliehyd '0 rams of cobalt acetate; ante 20grams =-of silver acetate? The operation was cjonduetedia the numericaldata being as follows.

20 cubic meters" per l i'.

Temperature of the bath Air supply Acetaldehyfleas pply lakes-mes 2Residuapgass=ret in to the 1 .5 99bit m ers pe vessel. ration oi= oxy anthe 8% y vo ume-- g s iu a e-ed-- inta the 8. Temperature 1 qndens e QCondensate as: btgiped in fiam A bet 8 Acetic anhydride et c ac d WaterAc sld hyde 'v ransie ni ti'eu r t V I 11 Anhydrid yield 71% -theoxidlzedeldehydez 3. Mixture in the reaction vessel? 84 kgs. ofdibutylfphthalate';

18 kgs. of acetidanhydtide;

2 kgs. of acetic acid,

1 kg. of acetaldehyde';

50 grams of cobalt'acetate, a id 500 grams of copper'acetate.

Th meation wascqnduc ecl s m x p e 1, the numerical data being asfollows'."

T mpera u e 9 ba Air supply es aldelisfler su pl 8... Residual gasesreturned to the vessel, V I p 6 Concentration of oxygen n; the;

lglasles introduced into the Temperature in condenser-7;; Colndensate asobtained in 011 er v used. Anhydride yield 78.6% 0

f aldehyde 4. Mixturein'the reaction vesselz 80 kgs. of allylbenzoate; I17 kgs. of acetic anhydrid'ej- 4 kgs. ofiacetic acid-.3, 1.5 kgs. ofacetaldehyde, 100 grams of cobalt acetate 100 grams of copper acetateiand 20 grams of silver acetates The operation was conducted as-inExample 1 the numerical data beingasffollo ss 'ei aozcumeseexersperm!15.5 kgs. per hour.

Temperature of the-bath Air supply Acetaldehyde supply "I so vesseConcentration of oxygen gases introduced int I bath.

Temperature in condenser 7 .10.

Condensate as obtained in cha n? Water Acetaldehyde 3 Transformationrate d Anhydride yield. v

- al ehyd 5. Mixture in thereaction vessel-t' vessel. Concentration ofoxygen in the? 5% byvolum. the

gags introduced into H a s v. Transformation rate ?;of-themtarting aliuAnhydride yield 88%"31 the'o'xidized aldehyde.

6. Mixture in thea'eaction vessel-z 75 kgs. of dibut yl maleate;-2'3k's1. off-ace it ihymiiiei i '1" kg:- of-acetie-acid,--

1 k of ae taldehyd e grams of cobalt ac'etateg and 50-grams Qf oprcetate.

The-operationi wa s conducted as he Example- 1, the numericald'ata beingas follows:

t mp rature of'the'bath Pressure Airsunsly mete s. 9. i.- at atmosphericAcetaldehyde supply 7 I No residual gases are returned to the vessel.- 7Partial pressure of oxygen in 53 mm. Hg.

ghel gases introduced ipto-tlie" V a 1. 1 a Transformation rate ---40%-of the starting al- .Q h-y ,4 Anhydride yield "[8032; of theoxidizedalehyd'e'a '7. Mixture in the reaction vessel:-

85 kgs. of butyl stearate';

13- kgs. of acetic anhydride,

1 kg. of aceticacid, V

1 kg. of acetaldehyde,

30 grams of cobalt aceta-te- 'siamsioi opner acetate: and?20--'gi"aI'-nsof sil-ver 'acetate The operation was was." v thenumerical databeing as follows. I

1 kg: '(ahoyeatnid'splieric pressure) per sq. cm. 20.cubi c meters.(calculated' at atmospheric fi esfl rper au s t y e ply (a q k. rhi nr.l Residual gases returned 'to-the"240- cub me rs F (at a vessel. relatiresell??? t-l 42-1 er 511cm. perhour?" q gages introduced 1857 1195 thefsta'r'tingil- Transformation rate t v 19g s t'li idi 2 07D F.l..e-0;4 sid- 11 dehyde,

21.5'kgs. per hour. 225 cubic meters per hr.

Temperature of the bath Air supply Residual gases returned to thevessel.

Concentration of oxygen in the gases introduced into the 6 by volume. 15

bath. Transformation rate 80% of the starting al- V dehyde. Anhydrideyield 85% of the oxidized aldehyde.-

' 9. Mixture in the reaction vessel:

SOkgs. of tributyl phosphatef 1'1 kgsaof'acetic anhydride,

2 kgs. of acetic acid,

1 kg. of .acetaldehyde,- 7

50 grams of silver acetate,

100 grams of nickel acetate, and

100 grams of copper acetate.

The operation was conducted as in Example 1, the numerical'data being asfollows:

55 C. 20 cubic meters per hr.

18 kgs. per hour. 240 cubic meters per hr.

"4% volume.

Temperature of the bath Air supply Acetaldehyde supply Residual gasesreturned to the vessel.

Concentration of oxygen in the gases introduced into the 82% of thestarting aldehyde.

90% of theoxidized alehyde.= In this operation, acetic acid was used asa solvent for washing the residual gases before their beingdiscarded-instead of using'the same ester as is employed as a diluent inthe bath, viz.

bath. Transformation rate Anhydride' yield tributyl phosphate.

10. Mixture in the reaction vessel: it 42 kgs. of ortho-tolyl benaoate,42 kgs. of tributyl borate, W 7 18 kgs. of acetic anhydride, 2 kgs. ofacetic acid;

1 kg. of aoetaldehyde, 50 grams of cobalt acetate, and 500 grams ofcopper acetate.

The operation was conducted as in Example 1, the numerical data being asfollows:

Temperature of the bath 50 C.

Air supply 10 cubic meters per hr.

Acetaldehyde supply 8 kgs. per hour.

Reslduall gases returned to the 160 cubicmeters per hr.

vesse Concentration of oxygen in the gases introduced into the b8.Transformation rate 68% of the aldehyde use Anhydride yield .a 75% orthe oxidized al- 'deliyde.

9% by volume.

11. The operation wastarnedout under the same'conditi'ons as in Example4, tetrahydrofurfuryl 'benzoate beingsubstituted for allyl benzoate',and the results were substantially the same. 12. The operation wascarried out under the same conditions as in Example 4, citronellylacetate being substituted iorallyl benzoate, and the results weresubstantially the same.

13. The" operation 'was' carried out under the *sameconditions as inExample 9, ethyl phenyl- 8 acetate being substituted for tributylphosphate, and the results were-substantially the same.

'14. The operation was carried out under the same conditions as inExample 9, diethyl dibromomalonate being substituted for tributylphosphate, and the results were substantially the same.

15. Mixture in the reaction vessel:

60 kgs oi ortho diethyl phthalate, 16 kgs. of glycerol triacetate, W

22 kgs. of butyric a hydride,

2 kgs. of butyric acid, I

1.2 kgs. of butyraldehyde, 200 grams of copper acetate, and 100 grams ofcobalt acetate.

The operation was conducted as in Example 1, the numerical data being asfollows:

0. 19 cubic meters per hr.

23.5 kgs. per hour. 161 cubic meters per hr.

Temperature of the bath Air supply Butyraldehyde supply Residual gasesreturned to the vessel Concentration of oxygen in the 6% by volume.

gases introduced into the bath. Temperature in condenser 7.. 15 C. Condeisate as obtained in cham- Butyric anhydride 75% by w ight. Butyricacid 8.65% by eight. Water 8.55% by weight. Butyraldehyde 7.8? byweight. Transformation rate 90. of the starting aldehyde. Anhydrideyield 90.5% of the oxidized aldehyde.

In this operation, butyric anhydride was used as a solvent for washingthe residual gases, instead of using the same esters as are employed asdiluents in the bath, viz. ortho-diethyl phthalate and glyceroltriacetate; r

16. Mixture in the reaction vessel:

78 kgs. of ortho-diheptyl phthalate, 19.5 kgs. of propionic anhydride,

2.5 kgs. of propionic acid,

1.4 kgs. of propionaldehyde,

180 grams of copper propionate, and 180 grams of cobalt propionate.

The operation was conducted as in Example 1, the numerical data being asfollows:

Temperature of the bath 65 C. Air supply 15 cubic meters per hr.Propionaldehyde supply 15 kgs. per hour. .Residuall gases returned tothe 150 cubic meters per hr.

vesse Concentration of oxygen in the 6.8% by volume.

gases introduced into the bath.

Temperature in condenser 7---- 12 C.

Colndersate as obtained in cham- 7 Propionicanhydride 66% by weight.Propionic acid 13.7% by weight. Water 9.1% by weight. Propionaldehyde11.2% by weight.

Transformation rate $652,621 the starting alc y e. Anhydride yield 84.6%of the oxidized V, aldehyde. 17. Mixturein the reaction vessel:

77 kgs. of ortho-dimethyl phthalate, ll kgs. of aceto-butyric anhydride8 kgs. of butyric anhydride,

3.5 kgs. of aceticanhydride,

1.7 kgs. of butyric acid,

0.8 kg. of acetic acid,

1.1 kgs. of butyraldehyde,

0.5 kg. of acetaldehyde,-

grams of copperacetate, and grams of cobalt butyrate. I

meager;

Aldehyde supply (equimolecular mixture of..butyraldehyde .and

prisin acetaldehyde) 8S- -Actaldehyde, 7.75 V ES. Residual -gasesreturned to the --180 cubic meter's peih'r.

. (vessel. 1;. .Cencntration of oxygen in the 5.8% by volume.

gaisgs "introduced into --the j\ a p M n I: -,1emperature,in condenser 7,1.0 C. fQode%sate-as obtained in cham b er Y Aceto-butyric anhydride"-62.4% by weight Butyric anhydride 5.45% by weight Acetic anhydride 10.1%by weight Butyric acid 1.55% by weight. Acetic acid 0.95% by weight. Wa1- 11.05% by weight. Butyraldehyde 4.1% by weight. Acetaldehyde 4.4% byweight.

91% of the starting bu- Transformation rates tyraldehyde.

87.5% of the starting acetaldehyde.

Total anhydride yield 97% of the oxidized al- Relative proportions ofthe obdehydes.

tamed anhydndes:

Aceto-butyric anhydride 80% by weight. Butyric anhydride by weight.Acetic anhydride 13% by weight.

It is remarkable that when mixed anhydrides, such as aceto-butyricanhydride, are prepared in accordance with my process, the yield ofmixed anhydride is substantially higher with respect to the total yieldof the corresponding simple anhydrides produced at the same time, suchas butyric and acetic anhydrides, than in processes used heretofore forpreparing such mixed anhydrides.

18. Although the esters are the preferred diluents to be employed inaccordance with my invention, this invention is not limited thereto andother substances answering the mentioned requirements may be used asdiluents, giving excellent results. As an example, there will bedescribed thereaiter an operation using a higher acid anhydride as adiluent.

Mixture in the reaction vessel:

78 kgs. of octanoic anhydride, 19 kgs. of acetic anhydride, 3 kgs. ofacetic acid,

1.35 kgs. of acetaldehyde,

. 200 grams of copper acetate,

50 grams of cobalt acetate, and 50 grams of silver acetate.

The operation was conducted as in Example 1,

the numerical data being as follows:

Temperature of the bath 54 C.

Air supply 16 cubic meters per hr. Acetaldehyde supply 13.6 kgs. perhour.

Residual gases returned to the 124 cubic meters per hr.

vessel. Concentration of oxygen in the 5.2% by volume.

gbasiilels introduced into the Temperature in condenser 7 Transformationrate 5 of the starting ale y e. Anhydride yield 77.8% of the oxidizedaldehyde.

20.35 kgs. per hour, com- M Bu tyraldehyde;=12.6

5 10 .wherc1.-two. -aldehydes-i re-.;used theucorresponding acids.Likewise, the term anhydride.appea-ring :ai-n ithewlainisnis intended torepresent eitherrone rssirnpleranhydride:incthe case where .cne;aldehyde is used, and in the case wheref-two: aldehydes :are vused, 1the; mixture consisting of .the mixed aanrhydridegand ritha :two' simple.anhydrides aresulting from oxidation of the two aldehydes. What-1J1claim is:

;}1..: -A process which comprises, continuouslyinproducing-angers.containing molecular oxygemand tal i-phatic -;a-lde1;1yde;:having from 2to ,4-carbon ;=.atoms,intQ arliquidi bath-icontaining aldehyde 'to beoxidized, the corresponding acid, the 121-11111187- -:dridezccrrespnnding-rto isaid .a-cid, metallic salts capable of acting asaldehyde oxidation catalyst and at least 75% of the total weight of saidliquid bath of an inert diluent selected from the group consisting of anester and an anhydride having a boiling point above 200 C. and mixturesof said ester and anhydride, which diluent is insoluble in and does notdissolve water, is miscible with the anhydride to be produced as well aswith the corresponding acid and when mixed with such acid and anhydride,such mixture is capable of dissolving the catalytic salts in theproportion required for catalysis of the reaction between the aldehydeand oxygen, and oxidizing such aldehyde at a temperature of about 40-100C. to produce the corresponding anhydride, the partial pressure of saidoxygen being lower than 76 mm. of mercury, said gas being introduced ata rate such that the residual gas removes from the bath substantiallyall of the anhydride, acid and water formed, while the diluent remainsin the bath.

2. A process in accordance with claim 1 employing an inert ester andupon oxidizing the aldehyde removing the gases from the bath, coolingsaid gases to separate anhydride therefrom and returning part of thegases directly to the bath.

3. A process in accordance with claim 2 which includes cooling the gasesat a temperature not substantially above 15 C., separating anhydridetherefrom and subjecting the remainder of the gas, after return ofuncondensed gases to the bath, to washing with a solvent to recover anyvaluable solute contained therein.

4. A process in accordance with claim 3 in which the washing is done bya circulating solvent which is of the same character as said diluentpassing through a cycle including a first zone for washing the residualuncondensed gases and a second zone for extracting the valuablesubstances therefrom, in which second zone the solvent containing saidsubstances is put into contact with fresh gas containing molecularoxygen flowing towards the oxidation bath so that such gas carries suchrecovered valuable substances and as such is introduced into said bath.

5. A process according to claim 4 in which the solvent circulatingthrough the cycle is subjected to variations of temperature so as to berelatively cold for washing the residual gases and relatively warm forextraction of the valuable substances therefrom.

6. A process according to claim 3 in which the solvent is the samesubstance as is used as a diluent in the oxidation bath.

7. A process in accordance with claim 1, in which said diluent is anester-formed from an acid and a hydroxy-compound, said acid beingselected from the class consisting of monobasic and polybasic inorganicand organic acids, said hydroxy-compound being selected from the classesconsisting of monobasic and polybasic alwhich concentration of aldehydein the bath is cohols and phenols. maintained at about 0.3 to 5% byweight of said 8. A process in accordance with claim 7 in bath. whichthe aldehyde is acetaldehyde and acetic LINO RIGONl anhydride isproduced. 5

9 A process accordance claim 7 in References in the me of patent whichthe bath contains by weight 15% to 90% UNITED STATES PATENTS of diluentNumber Name Date 10. A process in accordance with claim '1 in which thepartial pressure of oxygen in the 10 fi? "i 2i oxygen-containing gas isbetween 23 and 60 mm. 2225486 Reigharta 1940 of mercury, said gas beingsupplied to the bath at 2293104 Bmdworu; 1942 the rate of about 1 to 3cubic meters per hour per 2,575,159 Chassamg et ah Nov. 13 1951 liter ofbath.

11. A process in accordance with claim 7 in 15

1. A PROCESS WHICH COMPRISES, CONTINUOUSLY INTRODUCING A GAS CONTAININGMOLECULAR OXYGEN, AND ALIPHATIC ALDEHYDE HAVING FROM 2 TO 4 CARBONATOMS, INTO A LIQUID BATH CONTAINING ALDEHYDE TO BE OXIDIZED, THECORRESPONDING ACID, THE ANHYDRIDE CORRESPONDING TO SAID ACID, METALLICSALTS CAPABLE OF ACTING AS ALDEHYDE OXIDATION CATALYST AND AT LEAST 75%OF THE TOTAL WEIGHT OF SAID LIQUID BATH OF AN INERT DILUENT SELECTEDFROM THE GROUP CONSISTING OF AN ESTER AND AN ANHYDRIDE HAVING A BOILINGPOINT ABOVE 200* C. AND MIXTURES OF SAID ESTER AND ANHYDRIDE, WHICHDILUENT IS INSOLUBLE IN AND DOES NOT DISSOLVE WATER, IS MISCIBLE WITHTHE ANHYDRIDE TO BE PRODUCED AS WELL AS WITH THE